How to Measure Optimal Light Intensity for Indoor Growing?

Sunlight is like a magical energy source in the plant world, playing a crucial role in every leaf and flower in our homes. Without sufficient sunlight, even the most resilient plants can become limp and unhealthy.

Light provides the energy needed for photosynthesis and directly influences factors such as growth rate, leaf color, stem hardness, and flowering.

Simply put, light is like a nutritional meal for plants – too much or too little won’t do; it needs to be just right to ensure they grow healthy and beautiful.

Speaking of just right, this brings us to the issue of light intensity.

  • Each plant has its preferences; some thrive under bright sunlight, like sun-loving plants such as cacti, which require ample light to grow vigorously.
  • On the other hand, some plants prefer dappled light, akin to shy maidens in the forest, feeling more comfortable with gentle, scattered light rather than direct sunlight.

Accurately measuring and adjusting the light intensity in the indoor planting environment is as crucial as customizing a nutrition plan for each plant. With precise data, we can better simulate the lighting conditions found in nature, allowing each plant to thrive in its optimal light environment, growing happily and blossoming beautifully.

Table of Contents

Understand the Basic Concept of Light Intensity

To help indoor gardening enthusiasts better care for their houseplants, let’s discuss a few basic concepts related to light intensity simply and understandably!

Light Intensity: It’s like turning on a light in a room – how bright the light is represents the light intensity. In gardening, it measures the amount of visible light flux received per unit area, often expressed in units called lux (lx).

However, in plant growth research, we are more concerned with the portion of light that is effective for photosynthesis in plants.

Photoperiod: This is like setting a biological clock for plants. It refers to the duration of light and darkness alternation, such as 12 hours of light followed by 12 hours of darkness, which is a typical 12/12 photoperiod.

Different plants have different preferences for photoperiod lengths, which directly affects their growth stages, including flowering and dormancy.

Photosynthetic Photon Flux Density (PPFD): This measures the amount of light that is truly useful for plant growth. PPFD quantifies the number of photosynthetically active photons that reach a unit area of the plant’s surface per unit time.

Its unit is micromoles per square meter per second (μmol/m²·s). It’s crucial because only photons within specific wavelength ranges can stimulate plants to undergo photosynthesis and synthesize the nutrients they need.

Application of PPFD in plant growth: PPFD is like the “food quantity” for plants. When plant leaves receive sufficient PPFD, they can efficiently carry out photosynthesis, producing more energy and oxygen to promote growth.

Different plant species have varying PPFD requirements at different growth stages. For instance, vegetable crops often require higher PPFD to ensure yield, while some foliage plants may adapt well to lower PPFD levels.

Choosing the Right Light Measurement Tools

Light Measurement Tools: Lux Meter and Quantum Light Meter

  • Lux Meter is primarily used to measure the overall light intensity, which is commonly referred to as “illuminance.”
  • Quantum Light Meter is more specialized, focusing on measuring the portion of light that directly contributes to photosynthesis in plants—Photosynthetic Photon Flux Density (PPFD).

Handheld Light Meters/Light Quantum Meters

They are like mini detectives in your hand—compact and lightweight, allowing you to directly measure light intensity near plants or under LED grow lights. Simply aim the meter at the light source, press the button, and the display screen will show the light intensity value.

The advantage of handheld meters is their flexibility and convenience, making them particularly suitable for home gardening enthusiasts and small-scale greenhouse environments.

Hanging/Fixed Light Meters/Light Quantum Meters

If you have a large indoor farm or a large plant room at home, then this type of measurement tool is more suitable for you. These types of meters can be permanently fixed in one place, continuously monitoring the lighting conditions, and recording the changes in light intensity throughout the day, allowing you to understand the distribution and stability of light in the entire space at any time.

Smart Light Measurement Devices

Smart light measurement tools typically come with wireless transmission capabilities, allowing real-time data to be sent to a smartphone app or computer. They may even have the ability to automatically adjust the brightness and switch times of LED grow lights.

These intelligent devices eliminate the hassle of manually reading and recording data, making them especially suitable for tech enthusiasts and busy urban farmers.

How to Choose the Right Measurement Tool

When selecting a light measurement tool, consider the following aspects:

Functional Requirements: If you simply want to get a rough idea of the light intensity on your windowsill at home, a basic handheld light meter will suffice. However, if you are serious about indoor gardening, especially if you use LED grow lights, you should consider purchasing a quantum light meter to precisely adjust your light formula.

Measurement Accuracy: The more precise the device, the more accurate the measurements, but also the higher the price. Decide on the level of precision you need based on your requirements for light control.

Budget Considerations: Light measurement products on the market vary in price, ranging from basic models that cost tens of dollars to professional-grade ones that cost thousands of dollars. Choose a tool that fits your budget. Remember, cost-effectiveness is key.

Convenience and Expandability: If you plan to expand your cultivation scale or add automated facilities in the future, consider investing in smart devices that support remote monitoring and linkage control.

Implementing Light Intensity Measurement Steps

Positioning Measurement Points

Conducting light intensity measurements at different positions within the plant canopy is based on the fact that light attenuates as it passes through plant leaves and branches, and there are significant differences in light conditions between the interior and the edge of the canopy.

This means that the light intensity received by different parts of the plant is not uniform. To ensure that each part receives sufficient light, you need to measure at the following positions:

  1. Canopy Top: This is the position with the strongest light intensity, typically representing direct sunlight conditions.
  2. Canopy Middle: Here, the light may be shaded by upper leaves, reflecting the light conditions of middle-level leaves.
  3. Canopy Bottom: Leaves near the base usually receive the weakest light, as they are more likely to be shaded.

During measurement, place the light meter at the same plane as the leaves, mimicking the angle at which the leaves receive light, to ensure that the data accurately reflects the actual light environment of the plant.

Real-time Measurement and Recording

To obtain comprehensive light data, you need to conduct multiple measurements at different times of the day because the position of the sun and the intensity of daylight change over time. For example:

  1. Shortly after sunrise: At this time, the light is relatively soft, so observe the morning light intensity.
  2. Around noon: This is when sunlight is most intense, so measure the highest light intensity.

After sunset: If supplementary lighting is used (such as LED grow lights), record the artificial light intensity during the night.

Each measurement should be accompanied by recording the time and corresponding light intensity value. This allows you to plot a daily variation curve of light intensity, providing a basis for subsequent adjustments.

Analyzing Data and Adjusting

Upon receiving the light intensity data, you can compare it with the light requirements of different plants to assess if the current lighting conditions are suitable.

  1. Interpret the measurement results: Compare the measured light quantum flux density (PPFD) range with the light requirements of the plants. If the top of the canopy receives excessive light, it may cause sunburn on the plants; if the bottom receives insufficient light, it can affect plant growth.
  2. Adjust LED grow lights:

Height: If the bottom leaves receive insufficient light, try lowering the light fixture height to reduce light attenuation and increase bottom light intensity.

Angle: Adjusting the light fixture angle can alter light distribution, ensuring more uniform coverage over the entire canopy.

Light duration: Based on the measurement results, adjust the light duration accordingly to meet the daily light cycle requirements of the plants.

Continuous experimentation and fine-tuning are key to finding the optimal lighting settings. By regularly checking and recording changes in light intensity, you can dynamically optimize the lighting scheme to ensure that plants are always in the most suitable lighting environment, thereby promoting healthy and rapid growth.

Ideal Range of Light Intensity for Growing Plants Indoors

Green pothos

As a common indoor foliage plant, green pothos have good shade tolerance and thrive in relatively low light conditions. The ideal light intensity for green pothos is approximately 100-200 μmol/m²·s.

Even in low-light areas, green pothos can still grow, but if the light is too strong, signs of leaf burn may appear.


Although herbs can survive in partial shade conditions, providing relatively high light intensity is recommended to ensure their aroma and growth vigor. Generally, the ideal light intensity for herbs is between 300-600 μmol/m²·s.

Annual herbs like basil grow more vigorously in adequate light conditions, while perennial herbs like mint can adapt to lower light but still require some light stimulation to maintain good growth and aroma accumulation.

Leafy vegetables

Leafy vegetables like lettuce and spinach have relatively low light requirements but still need some light to ensure photosynthesis. The ideal light intensity for leafy vegetables is typically between 200-500 μmol/m²·s.

Bean sprouts, which grow rapidly and have a very short lifecycle, have relatively lower light requirements.


Cannabis is a light-sensitive plant, especially during the early growth and flowering stages, where light intensity is crucial for its growth and yield. To obtain high-quality cannabis, growers typically provide high-intensity light, with the ideal light intensity ranging between 400-1000 μmol/m²·s. Particularly during the seedling and flowering stages, light intensity should not be lower than 600 μmol/m²·s.


Although strawberries have some shade tolerance, to achieve abundant fruits and good flavor, the ideal light intensity should be between 300-800 μmol/m²·s, especially during the flowering and fruiting stages, when higher light levels are needed.


Tomatoes are typical sun-loving plants, and sufficient light intensity greatly impacts their growth, development, and yield. When grown indoors, the light intensity for tomatoes should be at least 500-1000 μmol/m²·s, particularly during the fruiting stage.

Higher light intensity during this stage aids in fruit ripening and sugar accumulation, thereby improving the quality and yield of tomatoes.

Get in touch with us!

From custom light planning, to tailored quotes, and everything in between, our team of horticulture experts are always ready to assist.

Please enable JavaScript in your browser to complete this form.
** Your privacy will be protected

Get Catalogue & Price List​

Please enable JavaScript in your browser to complete this form.
** Your privacy will be protected
Open chat
Inquiry Us
Hello 👋
Are you looking for LED Grow Lights?